The present invention relates to an apparatus for delivering intravenous drugs to a patient.
More particularly the invention is concerned with a drug infusion apparatus provided with a protection against drug overdose that could occur because of a free and uncontrolled delivery flow of the liquid drug from the apparatus to the patient.
A presently known infusion apparatus provides for electromechanical devices capable of applying a slow and progressive motion to a slider that in turn moves a plug in a plastic syringe, this latter being either of conventional design or purposely designed for liquid drug infusion.
These devices generally comprise a threaded shaft connected to a slider actuating the syringe plunger for injecting a drug into the patient through a small hose terminating with a needle that is permanently inserted under the skin.
An apparatus of this type is disclosed in Italian Utility Model No. 193915. In accordance with the teaching of Italian Utility Model No. 193915 a portable apparatus for delivering insulin to a patient comprises a housing for a conventional syringe, a slider acting on the syringe plunger, a threaded shaft for advancing the slider, an electric motor for rotating, the threaded shaft and an electronic control unit for controlling the apparatus operation.
A drawback of the apparatus disclosed by Utility Model No. 193915 comes from the fact that such apparatus results in being rather awkward and therefore unsuitable for portable use when it is realized for syringes capable of delivering drug amounts in the order of 20 cc. and larger, for example of the type used in ferro-chelating therapy.
To overcome the above discussed drawback, several devices have been proposed, one of them being disclosed in Italian patent application No. T092A000561 in the name of the present applicant.
The drug infusion device according to the above mentioned patent application comprises a pushing member or pusher made up by an axially movable threaded shaft for operating the syringe (plunger).
To this pusher there is directly fitted the plunger of a syringe filled with the drug to administer.
To this purpose the syringe is equipped with a removable stem that is used for sucking the drug and is then removed.
The device realized according to Italian patent application No. T092A000561 has a smaller size and can be used as a portable device.
However the above mentioned device has the shortcoming of a poor seal between the pusher and the syringe plunger which under certain circumstances could lead to the detachment of the pusher from the plunger.
More particularly, when for any reason the apparatus with the syringe containing the liquid is located a few tens of centimeters above the needle inserted under the skin, a drug can be delivered to the patient at an uncontrolled rate (xe2x80x9cfree flowxe2x80x9d).
Such uncontrolled rate flow is substantially caused by the pressure difference in the syringe due to the height difference between the syringe and the point at which the under-the-skin needle is located.
Thus a pressure difference (or overpressure) caused by the liquid column in the hose is present at the end near the needle.
Such pressure difference causes a larger downflow of the liquid and creates a lower pressure in the syringe chamber.
The resulting effect is further enhanced by the diameter difference between the hose and the syringe. Such lower pressure applied to the syringe plunger surface generates a force strong enough to attract the plunger and cause its detachment from the pusher. This way an uncontrolled or free flow of liquid is created that can cause a drug overdose resulting in serious trouble for the patient.
In this regard it is pointed out that in some countries the infusion devices are subjected to tests comprising a check of the protection degree ensured by the device against the free flow drawback.
It is therefore an object of the present invention to provide an apparatus for infusing drugs that is not affected by a free (uncontrolled) flow of liquid when a drug is being administered to a patient.
Another object of the present invention is to provide an infusion apparatus in which the syringe plunger can be easily fitted to the pusher and without causing slippages of the plunger that could cause drug leakages.
The above and other objects of the invention are achieved through an apparatus for drug infusion as claimed in the attached claims.
The present invention provides for mechanically locking the syringe plunger thanks to the elasticity of a plunger made of rubber for ensuring a proper seal in the plunger-pusher connection.